Feature: File uploads

[sorokya]

Fixes #27

Not sure if the web-component code also needs updated?

I added a demo for the new code. There's no styling for drag-over but it works.

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Summary by cubic

Adds file upload support with paste and drag‑and‑drop, including optional batch uploads and placeholder handling. Updates docs, types, and demo; bumps to v2.2.0.

• New Features
  • Paste or drag-and-drop files into the editor; a placeholder is inserted and replaced after async upload.
  • New fileUpload options: enabled, maxSize (default 10 MB), mimeTypes filter, batch mode.
  • onInsertFile callback accepts File or File[] and returns markdown string(s) to insert.
  • Images are auto-detected and use the syntax; failures replace placeholders with Upload failed.
  • TypeScript defs, README examples, and a website demo with a batch toggle are included.

^{Written for commit d22ef60. Summary will update automatically on new commits.}